This invention relates to displays made from electrochromic materials, and more particularly to such displays having an ion-conducting medium for ion transfer disposed between spaced layers of electrochromic material.
The term "electrochromic material" as used herein is defined in the literature, and is understood in the art to mean or to refer to a material responsive to the application of an electric field of a given polarity to change from a first stable persistent state in which it is essentially non-absorbtive of electromagnetic radiation in a given wave length region, and hence clear or substantially colorless, to a second stable persistent state in which it is absorbtive of electromagnetic radiation in the given wave length region, and hence colored. Once in said second stable state, it is responsive to the re-application of an electric field of the opposite polarity to return to its first stable state. As is well known in this art, the phenomenon is evidenced by a change in color, for example, the change in color from essentially clear to blue, and back to clear, obtained by electrically reducing thin films of tungsten (VI) oxides in a sulfuric acid electrolyte, as described in Journal of the American Chemical Society (Vol. 73, pp. 5427-5732, Nov. 1951) at p. 5429 in an article by Brimm et al. entitled =Sodium Potassium Tungsten Bronzes."
There are numerous examples in the art to which the present invention relates of fabrication of devices incorporating electrochromic materials. For example, devices in which one layer of electrochromic material is sandwiched with or without an ion-conducting material, such as an electrolyte, between two conductive electrode surfaces are shown in Talmey U.S. Pat. Nos. 2,173,141, 2,281,013 and 2,319,765, Deb et al. U.S. Pat. No. 3,521,941, and Deb U.S. Pat. No. 3,829,196. Devices incorporating two layers of electrochromic materials between two electrodes and separated by an ion conducting medium are shown in Brimm et al. supra, Giglia et al. U.S. Pat. No. 3,827,784, Witzke et al. U.S. Pat. No. 3,840,287, Bruesch et al. U.S. Pat. No. 3,971,624, Giglia U.S. Pat. No. 3,973,829, and Leibowitz U.S. Pat. No. 4,012,831.
Prior art devices which incorporate crystalline or solid state ion conducting media, for example, Deb et al. U.S. Pat. No. 3,521,941, Bruesch et al. U.S. Pat. No. 3,971,624, Jasinski U.S. Pat. No. 3,995,943, and Kasai U.S. Pat. No. 4,009,936 exhibit switching speeds from clear to colored to clear states, which are too slow for many display applications, for example, timepiece displays. Moreover, prior art devices which utilize concentrated acids in liquid or semi-solid form, for example, sulfuric acids as shown in Witzke et al. U.S. Pat. No. 3,840,287, also exhibit the same defects, and as well additional defects in that (1) the concentrated acid promotes the migration of anions from one layer of electrochromic material to another, and (2) the concentrated acid attacks, and ultimately destroys, conventional seals of the device.
Furthermore, when the insoluble form of cationic ion exchange resins is utilized, as disclosed in Bruesch et al. U.S. Pat. No. 3,971,624 (at col. 4, lines 27-29), as an ion conductive material between two layers of solid electrochromic materials, additional problems are encountered. For example, proper operation of the display requires continual uniform contact between both layers of electrochromic material and the cationic ion exchange resin. Cationic ion exchange resins disclosed for this purpose in the prior art, for example in Bruesch et al. supra, and Kissa U.S. Pat. No. 3,453,038 (at col. 5, lines 18-34), are in the form of insoluble sheets or insoluble powders. The sheet form is thick, and ultimately exhibits discontinuities in contact between two solid layers of electrochromic material, as well as exhibiting expansion when chemically wetted. Such sheet materials when chemically wetted expand significantly in volume causing physical separation of the substrates of the fabricated device, and consequent leakage of fluids contained therein. Resins fabricated in powder form are difficult to work with from a manufacturing standpoint, since the insoluble powder must first be dispersed in a binder before being placed between two layers of electrochromic material.
It is, therefore, the purpose of the present invention to provide a display device, incorporating spaced layers of solid electrochromic material, which is significantly less subject to degradation than prior devices, because no liquid electrolyte is employed, thereby obtaining improved sealing of the display construction, and having improved ionic transfer between the electrochromic layers due to the mobility of hydrated hydrogen ions and having improved switching speeds due to the blocking of passage of anions through the ion transferring medium.